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1978 DT250E Prokect - Ignition Coil observations and questions

  • johnste1960
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I have a 1978 DT250 project, Complete, decent shape, No Spark reported by seller.   I also have a 1977 DT250 parts bike (no engine)

I picked up a inline spark tester and when I installed and tested,  There was spark, but seemed weak and yellowish.  so I decided to check ignition coil as it was easy and I was waiting on points, condensor, and flywheel tool to be delivered.

The coil on the bike was a Mistibuishi F6T411 (yamaha part number 1M1-40271 7Z).  see attached photo of part from bike.

  The odd thing was that the part number on the parts lists for this coil was 1Y8-82310-40-00.  

When I searched the internet for the 1M1 number above,   I could not find any reference to this part number.  

When I bugged out the coil per the procedures in the manual, it checked out per all the specs I could find..   Note that the manual describes that the primary resistance cannot be checked as there is a diode in the circuit.F6T411:
Pri =  (RES CHECK) 2.1 Mohm with red to ground,  32 Mohm with black to ground
          (diode check)  0.55 V with red to ground. Open circuit with black to ground
Sec = 6.1 Kohms
Plug Cap = 5.4 KOHMS
 The confusion came to me when I noticed that the same coil was used on the 1977 DT250D.  However,  The manual for the 1977 says the primary resistance should be 1 ohm.  I pulled the coil from ten 1977 DT250D, see second picture

Doing a check on this coil showed it was fine per the DT250D specs I could find.  The manual for the 1977 did not show any diode and the testing revealed there was no diode either.   Problem was it was a different part number.  Again, when I searched for this on parts list (PARTZILLA)  no match was found,  however,  a internet search on the manufacturer part number showed it was a Hitachi Coil used on YZ250.  There seemed to be some sort of CDI coil as there was a part number CDI-CA 809 stamped on it.   reasearch seemed to show this was a Hitachi coil that was used on the YZ250.  Also note the difference in the resistance of the plug cap.   Note,  I suspect this was not the original coil for the 1977, but really did not care as that bike had no engine.   I am looking strictly to potentially use this as a substitute for the DT250ECm61-20:
Pri =  0.6 - 1 ohms.   Jumpy- probably my meter
Sec = 6.1 Kohms
plug Cap = 9.9 KOHMS

So both coils seem to be in working order, but here are my questions

1) Does anyone recognize the 1M1 coil part number off the 1978 bike.  Seems to be the right coil functionally, but does not have the correct Yamaha Part Number on it.
2) Does Yamaha not mark there parts with their own correct part number?
3) are ignition coils fairly universal,  That is in the case above, the coils off of the 1977 bike and 1978 bike seem to have correct resistance readings,  but one has the DIODE in it.   THe manual indicates that the diodes function is to prevent a motor from running in the reverse direction (backwards) which I was not even aware was possible.   I am guessing this is on a points setup and that CDI ignitions have an alternate mechanism to prevent this.   So if this is the case,  seems like bikes were produced many years without a diode and that these coils should be interchangeable.
4)  What is the significance of the different spark plug cap resistances?   Do different bikes have different values?    In some research,  I do see that 5000 and 10000 seem to be common but do not understand when and where to use which.   In reality,   I was always told that the resistance was only there to suppress RF which interfered with headsets and radios,  so as a result,  my expectation would be that NON RESISTOR plugs would be used on these types of bikes.  What is the advantage of using resistive plugs.
5) If you look at the two coils,   they appear to be the same physical shape except the 1978 coil has a n additional potted piece off one side which I assume is the DIODE.
16 May 2021 14:10 #1

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